1. Field of the Invention
This invention relates to a hollow body of a synthetic resin, a method for its injection molding and a mold for its molding, and more specifically, to a hollow body of a synthetic resin made by joining a pair of semi-hollow bodies along a joining space, a method for molding such a hollow body of a synthetic resin by primary and second injection molding and a mold used for carrying out the molding method. More particularly, it relates to a hollow body of a synthetic resin which is suitable for a gasoline tank to be mounted in a motor vehicle, a method for its injection molding and a mold for its molding.
2. Related Art
A molding method using an injection molding machine is known as one of the methods for molding a hollow body, or a hollow molded product of a synthetic resin. A mold used for carrying out such a method for injection molding is generally composed of a stationary mold and a slidable mold as set forth in, for example, Japanese Patent Publications Nos. JP-B-2-38377, JP-A-6-23789 and JP-A-6-246781. One primary semi-hollow body is formed in the stationary mold and the other primary semi-hollow body in the slidable mold. In primary molding, therefore, the slidable mold is set in the first position and a pair of primary semi-hollow bodies are so molded that each may have a butting or joining portion about its open end, and after they have solidified, the slidable mold is slid to the second position so that the primary semi-hollow bodies in pair may but each other at their butting portions. In secondary molding, a molten resin is injected into the joining space and the primary semi-hollow bodies in pair are joined to each other at their butting portions to form a hollow body of a synthetic resin.
The injection molding method as described above has the advantage of making it possible to automate the steps and produce a large quantity of hollow bodies, since a hollow body can be obtained by butting a pair of primary semi-hollow bodies formed by primary molding, and injecting a molten resin to fill the butted portion by secondary molding. The injection molding of a pair of primary semi-hollow bodies has features including the possibility of manufacturing even a hollow body having a complicated shape. It is, however, likely that the joined portion may be low in strength. FIGS. 6A though 6C are sectional views showing a part of an example of a hollow body obtained by the injection molding method as described above, and reference is made to FIG. 6A for the explanation of the reason for low joining strength. In FIG. 6A, A′ denotes a first primary semi-hollow body and B′ a second primary semi-hollow body, and the first and second primary semi-hollow bodies A′ and B′ are united together by the molten resin injected for secondary molding to fill the joining space S′. The joining space S′ is defined about the butt ends T′ of the open ends of the first and second primary semi-hollow bodies A′ and B′ and when the molten resin for secondary molding is injected to fill the joining space S′, therefore, the first and second primary semi-hollow bodies A′ and B′ are joined by the joining space S′ alone and the notch portions, or the butt ends T′ are not welded together. Accordingly, the welding force is low. It may be thought of to reduce the thickness of the butt ends T′, but as a low injection pressure for the injection of the molten resin for secondary molding into the joining space S′ gives a low welding or bonding force, it is necessary to inject it with a reasonably high injection pressure. Its injection with an injection pressure required for welding causes the deformation of the thin butt ends T′ and the leakage of the molten resin into the inside. For these reasons, the butt ends T′ have a specific thickness.
While there is no problem if only a compressive force acts between the butt ends T′ not joined together, the action of, for example, an external tensile force causes the separation of the butt ends T′ from inside. Then, cracking starts at the separated portion and extends to the joining space S′, as is generally the case with the destruction of a material. For these reasons, the structure as described above is unsuitable for any use calling for joining strength, for example, a gasoline tank mounted in a motor vehicle for which even an accident has to be anticipated.
Therefore, various attempts have so far been made to realize an elevation of joining strength. For example, JP-A-10-16064 proposes a joint shaped as shown in FIG. 6B. In this joint, the butt ends T′ of first and second primary semi-hollow bodies A′ and B′ are formed in an inclined way from a corner of a joining space S′. The joint proposed by JP-A-6-246781 is made by flanges formed integrally at the open ends of first and second primary semi-hollow bodies A7 and B′ during primary molding and connected with a resin R′ during secondary molding, as shown in FIG. 6C.
The butt ends T′ of the joint shown in FIG. 6B are more resistant to deformation by the pressure of secondary molding, since they are formed in an inclined way from the corner of the joining space S′. The degree to which the pressure of secondary molding acts upon the butt ends T′ is lower, since they are formed at the corner. As the butt ends T′ are not joined to each other, however, a tensile force acting upon that region causes the separation of the butt ends T′ and allows cracking to extend easily to the joining space S′, as explained with reference to FIG. 6A. In other words, the shape of the joint still fails to solve the problems explained with reference to FIG. 6A. On the other hand, the first and second primary semi-hollow bodies A′ and B′ shown in FIG. 6C can be said to have a high joining strength, since their flanges are joined to each other by the secondary molding resin R′ like a rivet. A third mold is, however, specifically required for secondary molding and is likely to add to the cost. Moreover, the outward appearance is not always a desirable shape.